1. Field of the Invention
The present invention relates to a machine tool with a pivotal spindle head, and particularly to a spindle head pivoting machine tool such as a multi-axis machining center.
2. Description of Relevant Art
Conventional machine tools of a spindle head pivoting type include a spindle head pivoting holder, which has a spindle head pivotally mounted thereto. Also the spindle head pivoting holder is pivotally mounted for rotation by a support shaft to a spindle head supporting base member that is constituted with ram, saddle, etc. to serve as a shaft carrier. The rotation of the spindle head pivoting holder is implemented by a gear train such as by a warm wheel mounted to the spindle head pivoting holder and a warm mounted to the spindle head supporting base member end, or by a sector gear mounted to the spindle head pivoting holder and a double pinion to the spindle head supporting base member end.
However, in arrangement having gears, such as warm wheel and warm or sector gear and double pinion, meshing with each other to rotate the spindle head pivoting holder, the structure is complicated, which constitutes a difficulty to increase the rigidity of a drive system because of the power transmission mechanism using gears.
As a solution to this problem, there has been disclosed in Japanese Patent Application Laid-Open Publication No. 10-43976 a spindle head pivoting mechanism in which a spindle head supporting base member (FIG. 5) has a lateral slider provided thereon to be driven by a feed screw in a lateral direction (L in FIG. 5), to a lateral position (x indicated by vertical line segments crossing the direction L in FIG. 5). The lateral slider has a vertical slider engaged therewith, which is movable for linear reciprocation (along a vertical line segment at the lateral position x) in the height direction of a chord of a rotation (at an angle xcex8 about a center Ct of a circle A with a radius R). The spindle head supporting base member further has a support shaft (at the center Ct of the circle A), which supports a base end (at the center Ct) of a spindle head pivoting holder (as the radius R) to be rotatable thereabout. The spindle head pivoting holder is pivotally secured at a distal end thereof (as a point xe2x80x98axe2x80x99 on the circle A) via an engagement piece to the vertical slider.
The lateral slider (the vertical line segment at the position x) is subjected to a great magnitude of moment (about a crossing point x between the line segment and the direction L) due to a load acting thereon at the distal end (the point xe2x80x98axe2x80x99) of the spindle head pivoting holder, and needs to have a great rigidity and strength. Moreover, because xcex8=sinxe2x88x921(x/R), the amount of lateral movement (xcex94x2 per angular unit xcex94xcex8) becomes smaller (than xcex94x1), as the angle (xcex8) increases (from an upright position xcex81 to a driven position xcex82), and has a decreased feed precision.
The present invention is made with such points in view. It therefore is an object of the present invention to provide a machine tool with a pivotal spindle head free of consideration to undue moment. It also is an object of the present invention to provide a machine tool with a pivotal spindle head that has an increased lateral movement, as a rotation angle increases, and has an increased feed precision.
To achieve the object, according to an aspect of the invention, there is provided a machine tool with a pivotal spindle head, comprising a movable base member, a spindle head holder pivoted on the base member, a linear drive mounted to the base member, a linear guide mounted to the spindle head holder, and an engagement member engaged at a substantially identical part thereof with the linear drive and the linear guide.
According to the aspect of the invention, because the engagement member is engaged at the substantially identical part with both the linear drive and the linear guide, various forces acting on the engagement member from the drive and guide do not produce undue moment. Further, because the linear drive extends in a tangential direction of a circle to be described about a pivotal point of the spindle head holder, this holder has an increased linear movement, as a rotation angle increases, and has an increased feed precision.